Flat-plate x-ray detectors currently have useable surface areas of up to 43×43 cm2. In order to manufacture these sizes it is necessary to manufacture the basic component, the plate, which consists for example of amorphous silicon, as a large surface. Since production machines are not designed for such sizes, in the prior art several plates are joined together and are bonded to one another on a substrate, for example a glass substrate. This bonding is also known as butting. A large-area flat-plate detector can for example consist of two or four individual plates. For example, the Pixium 4600 detector from Trixell consists of four individual plates made of amorphous silicon.
A problem entailed with butting is that the points where the individual plates are joined exhibit different sensitivity behavior and a variable diffusion of light, resulting in artifacts on the x-ray image. This is in particular also due to the cutting edges, which are never perfectly smooth. A hollow space between the plates is normally 20 μm to 60 μm wide and is ideally filled with adhesive. However, this ideal situation never pertains totally, rather there are air pockets and areas with excess adhesive. The plates can also be bonded on the substrate slightly canted from one another. The butting structures can extend over an area of 1.5 mm, which corresponds to ten pixels on each side of the plate. Thus it is clear that the butting structure can have a highly disruptive effect in an x-ray image.
Until now an average width of the butting structure has been assumed, and an interpolation of the pixels across this width is performed in the finished x-ray images. To a certain extent this interpolation is “fixed”, i.e. regardless of whether the butting structure in a particular detector used is actually as big or bigger. Nor are variations in the width of the butting structure within the detector taken into account, rather the interpolation width for the whole image is the same.
It is known from U.S. Pat. No. 5,528,043 to make a correction in a flat-plate x-ray detector for gaps between different areas of the detector (which correspond to the butting structure), whereby an interpolation of six nearest neighbors is performed in the case of column or row defects. Here too the interpolation does not take into account whether the butting structure is actually as big or bigger in a particular detector being used.
From US patent application US 2003/0169847 A1 an x-ray system is known in which four CCDs are combined to form one x-ray detector. The resulting butting structure is identified using an algorithm by detecting pixel image values corresponding to the butting structure, and is corrected by means of interpolation.
From EP 1 467 226 A1 structures for an x-ray machine are known in which individual sections of the image overlap. Thus this does not involve conventional butting. In the overlap area interpolations can be used for smoothing the image.
From DE 101 35 427 A1 it is known to infer the butting structure in a flat-plate x-ray detector indirectly by recording a sudden local grayscale change. Corresponding corrections are made in the grayscale values.